Body fat percentage is an estimate of the fraction of the total body mass that is adipose tissue (or referred to as Fat Mass), as opposed to lean body mass (muscle, bone, organ tissue, blood, and everything else) or referred to as Fat Free Mass. This index is often used as a means to monitor progress during a diet or as a measure of physical fitness for certain sports, such as body building. It is more accurate as a measure of excess body weight than body mass index (BMI) since it differentiates between the weight of muscle mass and that of the fat mass while BMI lump all masses into one figure. However, its popularity is less than BMI because equipment required to perform the body fat percentage is not readily available and skills are required to perform the measurement. Even when measured by a skillful person, there are factors that contribute to a significant margin of error.

Total body fat percentage consists of essential fat and storage fat. Essential fat is that amount necessary for maintenance of life and reproductive functions. The percentage for women is greater than that for men, due to the demands of childbearing and other hormonal functions. Essential fat is 2-5% in men, and 10-13% in women. Storage fat consists of fat accumulation in adipose tissue, part of which protects internal organs in the chest and abdomen. Again, women have slightly more than men.
The minimum recommended total body fat percentage exceeds the essential fat percentage value reported above.[2]

According to Health Check Systems,[2] The American Council on Exercise[3] has categorized ranges of body fat percentages as follows:

Description

Women

Men

Essential fat

12-15%

2-5%

Athletes

16–20%

6–13%

Fitness

21–24%

14–17%

Acceptable

25–31%

18–25%

Obese

32%+

25%+

Note that the essential fat values in the chart above are lower than the recommended minimum body fat percentage levels. A small amount of storage fat is required to be available as fuel for the body in time of need.[4] It is unclear whether falling in a particular category of these body fat percentages is better for one's health than any other, but there seem to be enhancements in athletic performance as one nears the ideal body fat percentage range for one's particular sport. The leanest athletes and bodybuilders typically compete at levels of about 5-8% for men, and 10-15% for women.[5] Getting to this level usually requires specific and carefully monitored variations in food, sodium and fluid intake, and also a carefully planned and implemented exercise program. It can be dangerous to maintain a body fat percentage at the low end of this range for more than a few days or a few hours. - "The New York Body Building Journal, 2004, June 9th"

There are several more complicated procedures that more accurately determine body fat percentage. Some, referred to as multicompartment models, can include DXA measurement of bone, plus independent measures of body water (using the dilution principle with isotopically labeled water) and body volume (either by water displacement or air plethysmography). Various other components may be independently measured, such as total body potassium.

In addition, the most refined method, in-vivo neutron activation, can quantify all the elements of the body and use mathematical relations among the measured elements in the different components of the body (fat, water, protein, etc.) to develop simultaneous equations to estimate total body composition, including body fat. This is the most accurate method.[How to reference and link to summary or text]

Prior to the adoption of DXA, the most accurate method of estimating body fat percentage was to measure that person's average density (total mass divided by total volume) and apply a formula to convert that to body fat percentage.

Since fat tissue has a lower density than muscles and bones, it is possible
to estimate the fat content. This estimate is distorted by the fact that muscles and bones have different densities: for a person with a more-than-average amount of bone tissue, the estimate will be too low. However, this method gives highly reproducible results for individual persons (± 1%), unlike the methods discussed below, which can have an error up to ±10%.[6] The body fat percentage is commonly calculated from one of two formulas:

Brozek formula: BF = (4.57/ρ − 4.142) × 100

Siri formula is: BF = (4.95/ρ − 4.50) × 100

In these formulas, ρ is the body density in kg/L. For a more accurate measurement, the amount of bone tissue must be estimated with a separate procedure. In either case, the body density must be measured with a high accuracy. An error of just 0.2% (e.g. 150 mL of trapped air in the lungs) would make 1% difference in the body fat percentage.

One way to determine body density is by hydrostatic weighing, which refers to measuring the apparent weight of a subject under water, with all air expelled from the lungs. This procedure is normally carried out in laboratories with special equipment.

The weight that is thus found will be equivalent to the body's weight in air, minus the weight of the volume of water which that object displaces. The following formula can be used to compute the relative density of a body: its density relative to the liquid in which it is immersed, based on its weight in that liquid:

where is relative density, is the weight of the body, and is the apparent immersed weight of the body. Absolute density is then determined from the relative density, and the density of the liquid. Because the density of water is very close to one, when density is computed relative to water, for many purposes it may be treated as absolute density.

Note that it is unnecessary to actually weigh a body under water in order to determine its volume, density or, for that matter, its weight under water. Volume can be easily determined by measuring how much water is displaced by submerging that body.

For a human body, a vertical tank which has a uniform cross-section-area, such as a cylinder or prism, can be used. As the subject submerges and expels air from the lungs, the rise in the water level is measured. The water level rise, together with the interior dimensions of the tank, determine the displaced volume.

Nevertheless, the equipment to actually weigh people under water exists, and some organizations, such as universities and major fitness centers, have it.

It is also possible to obtain an estimate of body density without directly measuring under water weight, and without directly measuring water displacement, either. What is required is a swimming pool or other tank where the subject can be fully immersed. The idea is to balance the body with a buyoant floatation device of a suitable mass and volume, such that the body plus floatation device neither sink nor float. The viability of this method rests in choosing a floatation device which has some convenient attribute that makes it possible to determine its volume easily: it is small, regularly shaped, and perhaps manufactured to a specific volume. From the volume and mass of the balancing floatation device, and the mass of the body, the volume and density of the body can be determined.

A person who neither floats nor sinks with empty lungs in water would have a density of approximately 1 kg/L (the density of water) and an estimated body fat percentage of 43% (Brozek) or 45% (Siri), which would be extremely obese. Persons with a lower body fat percentage would need to hold some kind of floatation device, such as an empty bottle, in order to keep from sinking. If the floatation device has mass m and volume v, and the person has a mass M, then his or her density is

where is the density of water [0.99780 kg/L at 22 °C (72 °F)]. For example, a person weighing 80 kg needs to hold a floater with a volume of 4.5 L and a mass of 0.5 kg has a density of 1.05 kg/L and hence a body fat percentage of 21%. Note that both the Brozek and Siri formulas are claimed to give systematically too high body fat percentages.[7]

A simpler version of the above formula can be derived by making two assumptions, and one small algebraic change. Firstly, the density of water can be taken to be 1 kg/L, which is more than accurate enough for the purposes. Secondly, the mass of light floation device such as an empty plastic bottle is tiny and so the term is negligible: if this assumption is invalid, it can easily be compensated for, as described below. Thirdly, the numerator and denominator can be multiplied by M, finally yielding

.

Note the similarity of this formula to that given earlier for relative density, except that masses are substituted for weights. The term also represents mass: the mass of water that was displaced by the floatation device to compensate the weight of the body in the liquid. That mass is actually where was taken to be one.

For example, an 80 kg person holding a 4 L floater of negligible mass has a density of 80/76 or about 1.05. Note that this is the same result as with the 4.5 L floater weighing 0.5 kg, using the more complicated formula. The reason is that if the floater has non-negligible mass, this mass can simply be subtracted from its volume to obtain an effective volume. An 8 L floater weighing 4 kg provides the same buyoancy as a 4 L floater of negligible mass. It can be visualized as a 8 L volume that is half-filled with water. The half that is filled with water can be removed from consideration.

For the above reasons, a light bottle partially filled with air makes a convenient floater, since the amount of air in it can be adjusted yet accurately measured. The measurement begins with a bottle completely filled with water. Some of the water is poured out into a collecting container, the bottle is sealed, and the subject is asked to perform a submersion, air expelled from the lungs, using that bottle as a floater. If the subject sinks, a small amount of water is removed from the bottle into the collecting container, and the experiment is repeated. If the subject floats, some water is returned from the collecting container to the bottle. When the subject finally achieves buoyancy equal to his or her weight (neither floats nor sinks), the amount of air in the bottle is determined by measuring how much water was poured into the collecting container, and the formula can be applied, where the variable is taken to be the volume of air in the bottle.

The Bioelectrical impedance analysis (BIA) method is more affordable but less accurate way to estimate body fat percentage. The general principle behind BIA: two conductors are attached to a person's body and a small electrical charge is sent through the body. The resistance between the conductors will provide a measure of body fat, since the resistance to electricity varies between adipose, muscular and skeletal tissue. Fat-free mass (muscles) is a good conductor as it contains a large amount of water (approximately 73%) and electrolytes, unlike fat which is anhydrous and a poor conductor of electrical current. Factors that affect the accuracy and precision of this method include instrumentation, subject factors, technician skill, and the prediction equation formulated to estimate the Fat Free Mass. Criticism of this methodology is based on where the conductors are placed on the body; typically they are placed on the feet, with the current sent up one leg, across the abdomen and down the other leg. As technician error is minor, factors such as eating, drinking and exercising must be controlled since hydration level is an important source of error in determining the flow of the electrical current to estimate body fat. As men and women store fat differently around the abdomen and thigh region, the results can be less accurate as a measure of total body fat percentage. Another variable that can affect the amount of body fat this test measures is the amount of liquid an individual has consumed before the test. As electricity travels more easily through water, a person who has consumed a large amount of water before the test will measure as a lower body fat percentage. Less water will increase the percentage of body fat. Bioelectrical impedance analysis is available in a laboratory, or for home use in the form of body fat scales and hand held body fat analyzers.

There exist various anthropometric methods for estimating body fat. The term anthropometric refers to measurements made of various parameters of the human body, such as circumferences of various body parts or thicknesses of skinfolds. Most of these methods are based on a statistical model. Some measurements are selected, and are applied to a population sample. For each individual in the sample, the method's measurements are recorded, and that individual's body density is also recorded, being determined by, for instance, under-water weighing, in combination with a multi-compartment body density model. From this data, a formula relating the body measurements to density is developed.

Because most anthropomorphic formulas such as the Durnin-Womersley skinfold method, the Jackson-Pollock skinfold method, and the US Navy circumference method, actually estimate body density, not body fat percentage, the body fat percentage is obtained by applying a second formula, such as the Siri or Brozek described in the above section on density. Consequently, the body fat percentage calculated from skin folds carries the cumulative error from the application of two separate statistical models.

These methods are therefore inferior to a direct measurement of body density and
the application of just one formula to estimate body fat percentage. One way to regard these methods is that they trade accuracy for convenience, since it is much more convenient to take a few body measurements than to submerge individuals in water tanks.

The chief problem with all statistically derived formulas is that in order to be widely applicable, they must be based on a broad sample of individuals. Yet, that breadth makes them inherently inaccurate. The ideal statistical estimation method for an individual is based on a sample of similar individuals. For instance, a skinfold based body density formula developed from a sample of male collegiate rowers is likely to be much more accurate for estimating the body density of a male collegiate rower than a method developed using a sample of the general population, because the sample is narrowed down by age, sex, physical fitness level, type of sport, and lifestyle factors. On the other hand, such a formula is unsuitable for general use.

The skinfold estimation methods are based on a skinfold test, whereby a pinch of skin is precisely measured by calipers at several standardized points on the body to determine the subcutaneous fat layer thickness. These measurements are converted to an estimated body fat percentage by an equation. Some formulas require as few as three measurements, others as many as seven. The accuracy of these estimates is more dependent on a person's unique body fat distribution than on the number of sites measured. As well, it is of utmost importance to test in a precise location with a fixed pressure. Although it may not give an accurate reading of real body fat percentage, it is a reliable measure of body composition change over a period of time, provided the test is carried out by the same person with the same technique.

Skinfold-based body fat estimation is sensitive to the type of caliper used, and technique. This method also only measures one type of fat: subcutaneous adipose tissue (fat under the skin). Two individuals might have nearly identical measurements at all of the skin fold sites, yet differ greatly in their body fat levels due to differences in other body fat deposits such as visceral adipose tissue: fat in the abdominal cavity. Some models partially address this problem by including age as a variable in the statistics and the resulting formula. Older individuals are found to have a lower body density for the same skinfold measurements, which is assumed to signify a higher body fat percentage. However, older, highly athletic individuals might not fit this assumption, causing the formulas to underestimate their body density.

There also exist formulas for estimating body fat percentage from an individual's weight and girth measurements. For example, the U.S. Navy Circumference method compares abdomen or waist and hips measurements to neck measurement and height,[8] and other sites claim to estimate one's body fat percentage by a conversion from the body mass index.

The U.S. Marine Corps and U.S. Army also rely on the Height and Circumference method. For males, they measure the neck and waist just above the navel. Females are measured around the hips, waist, and neck. These measurements are compared to a height/weight chart with age factored in as well. This method is used because it is a cheap and convenient way to implement a body fat test throughout the entire Department of Defense. This method poses a particular threat of inaccuracy because one can hold one's stomach in more if needed to pass the requirements, and/or flare the neck out and make it bigger resulting in a lower body fat percentage.

Due to different body compositions, those with larger necks have an advantage over those with smaller necks.